1. Field of the Invention
The present invention relates to zoom lens systems.
2. Description of the Related Art
Image pick up apparatuses (cameras), such as video cameras and digital still cameras, including solid-state image pickup elements require small zoom lenses with a high zoom ratio. This type of camera includes various optical elements, such as a low-pass filter and a color correction filter, between a rearmost portion of a lens and a solid-state image pickup element. Therefore, a zoom lens included in this type of camera is required to have a relatively long back focus.
In addition, a zoom lens included in a camera having a solid-state image pickup element for capturing a color image is required to have a high degree of telecentricity at the image side to avoid color shading. A negative-lead zoom lens is known as a small zoom lens having a long back focus and a high degree of telecentricity at the image side. In the negative-lead zoom lens, a lens unit having a negative refractive power is positioned in front (near the object side).
Japanese Patent Laid-Open Nos. 05-249374 (corresponding US patent: U.S. Pat. No. 5,381,268), 2003-131128, and 2004-102211 (corresponding US patent: US 2003/0210471) discuss a negative-lead zoom lens having a long back focus. The negative-lead zoom lens includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power in order from the object side to the image side. The negative-lead zoom lens performs zooming by moving both the first and second lens units.
Japanese Patent Laid-Open Nos. 2001-66503 (corresponding US patent: U.S. Pat. No. 6,545,819) and 2001-281545 (corresponding US patent: U.S. Pat. No. 6,498,687) discuss a negative-lead, three-unit zoom lens having a high degree of telecentricity at the image side. The negative-lead, three-unit zoom lens includes a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power in order from the object side to the image side.
Japanese Patent Laid-Open Nos. 2001-66503 and 2001-281545 discuss a small lens system having an achromatic structure in which a first lens unit that is nearest to the image side includes a single negative lens and a single positive lens.
Japanese Patent Laid-Open No. 2002-55278 (corresponding US patent: US 2003/0058549) discusses a three-unit zoom lens having a small number of lenses. The three-unit zoom lens includes a first lens unit in which a lens with a negative refractive power has an aspherical surface on each of the object side and the image side.
The overall size of the negative-lead zoom lens can be effectively reduced by increasing the refractive power of each lens unit. However, if the refractive power of each lens unit is simply increased, aberrations largely vary during zooming, and it becomes difficult to obtain a high optical performance over the entire zooming range. For example, as the field angle and the zoom ratio are increased, aberration variations during zooming are also increased. Therefore, it becomes difficult to obtain high optical performance over the entire zooming range.
Accordingly, to reduce the overall size of the negative-lead zoom lens while increasing the field angle and the zoom ratio, adequate settings of lenses included in the lens units are required. In particular, the lens structure of the first lens unit having a negative refractive power is important to reduce aberration variations during zooming and to obtain a high optical performance in the entire zooming range. If, for example, the lens structure of the first lens unit is not adequate, a chromatic aberration and variation thereof during zooming are increased and it becomes difficult to obtain a high optical performance over the entire zooming range.